With more people wanting to use natural filtration for their tanks, we are going to look at the two main types of units that you can put on your system: Chaeto reactors (or "algae reactors") and algae turf scrubbers (ATS). We won’t be looking at refugiums however, since those have mostly a different purpose. This will be a multi-part post; the next post will start with the basics, so if you’d like anything in particular to be covered, let us know.

All macroalgae operate basically the same, chemically. They all use light, photosynthetically, to absorb nutrients from the water (i.e., filtering) and to grow biomass. Just like trees. The differences between types of macroalgae are in the physical structure of the macroalgae growth and the way the structure affects nutrient absorption speed, which means filtering. Here are the main differences as far as aquarists are concerned:

Chaeto: Pronounced KAY-toe. Chaeto is the nickname for Chaetomorpha, and it looks like a green dishwasher cleaning pad. It has no "roots" and thus does not attach to solid surfaces. It grows in saltwater only, and is not eaten by many fish.

Green Hair Algae: Includes Cladophora "angel hair" and Ulva "Easter basket" types. It has "roots" which attach to solid surfaces. It grows in freshwater and saltwater, and is eaten by almost all herbivores.

Slime: A solid algal growth, bright green to brown to black in color, that attaches to solid surfaces but not very securely.

Chaeto Reactor: A device that has water running through it, with chaeto growing in it. Also known as an "algae reactor". A chaeto reactor does not allow air to enter; only water, and these reactors usually have a lid attached with screws to keep water in and air out.

Algae Scrubber: Also called a Turf Scrubber, or Algal Turf Scrubber (ATS). A device that allows air and water to interact to create a turbulent air/water interface like waves on a beach; it grows green hair algae or slime that attaches to solid surfaces.

Reactors and scrubbers are different from refugiums; a refugium (“fuge”) is a space in a sump where macroalgae is placed, and a light is put over it. Refugiums have very slow flow, and very low light penetration, compared to reactors or scrubbers. You could modify a refugium to be a reactor, and with more mods you could make it a scrubber. But then it would no longer be a refugium.

All oceans, reefs, lakes and rivers are naturally filtered by photosynthesis. This means that algae does all the filtering of these waters. This is why algae is at the base of the entire aquatic food chain, and why algae biomass dwarfs the biomass of all aquatic animals combined. But for algae to absorb nutrients out of the water, the algae must grow. And to absorb nutrients faster, the algae must grow faster.

Next we will look at what makes different types of macroalgae absorb nutrients differently.

Now for some basic differences; more detailed differences will be in subsequent posts.

The first and maybe most important difference is that chaeto reactors grow only in saltwater (fish only, or fish with live rock, or reef) whereas algae scrubbers grow (filter) in both saltwater and freshwater. Growing = filtering. But even if you are exclusively freshwater, understanding the differences between reactors and scrubbers enables you to optimize a system for your tank. There have not been any experiments of chaeto in brackish water however.

A second difference is size; a chaeto reactor needs to be much larger than an algae scrubber. Many saltwater tanks have large sumps, and even dedicated fish rooms, so this may not be an issue. Through experiential results of individual aquarists running chaeto reactors over the last few years, and through many thousands of aquarists running algae scrubbers over the last ten years, it has been observed that a chaeto reactor needs to be 4 to 8 times the physical size of an algae scrubber to provide the same rate of filtering capacity (rate of nutrient removal).

A third difference is seeding; a chaeto reactor needs to be seeded with a small amount of chaeto, either from another aquarium, reactor, or from your last harvest (i.e., you don’t harvest all of it), whereas an algae scrubber will self-seed from invisible algal cells in the water. When self-seeding, algae scrubbers usually start out with a slime type of growth, and this sometimes progresses on to a green hair algae growth, depending on the nutrients in the water.

A fourth difference is in how you clean (harvest). For a chaeto reactor, you disassemble the reactor usually by unscrewing several screws on the top of the container, and then by pulling out a tube or frame from the container; the chaeto growth is then removed from the frame and the frame is replaced back into the container, and the lid and screws are put back into place. Since chaeto does not attach to a surface, you often get broken chaeto pieces that flow into your tank or sump when you harvest; a filter screen in the reactor can reduce this.

For an algae scrubber, cleaning (harvesting) varies on what design it is; freshwater versions will usually be taken to a sink for the cleaning because of the thin and slimy growth (saltwater versions can also be cleaned in a sink, but are sometimes harvested in-place). A horizontal river design will have a light that you lift up off of the container, and a screen that you remove from the container. A waterfall design will have a screen that you remove from a pipe; sometimes the whole pipe is removed, and sometimes the pipe is in a container that you need to open first. A bubble upflow design has at least part of the container under water, which you lift out of the water. And for all algae scrubbers, since the growth is attached to a surface, broken floating algae pieces are not common when you harvest on a proper schedule. Bubble upflow scrubbers almost never detach because the growth is supported by the water.

A fifth difference is fish feeding; by feeding your fish from the growth, the fish eat naturally and you don’t have to buy and add food to the water (which creates nutrients). Very few if any aquarium animals eat chaeto, so the only option is to remove the chaeto and either throw it away or give it to a friend. For algae scrubbers, it depends on the growth: Slime (although full of absorbed nutrients from the water) is usually not eaten by aquarium fish and thus is scraped off and thrown away or used as garden fertilizer. Green hair algae however is eaten by almost all herbivore fish and many snails (it’s their nature food), and thus some of the growth can be fed back to the fish, especially in freshwater where algae scrubbers almost always grow this type of growth.

A sixth difference is overgrowth of algae on the lights. Chaeto reactors usually have a large surface area light (such as a long coiled light strip), and the illumination from these is not enough to “burn” off algae growth on the surface of the clear wall (this growth reduces illumination output). So you will need to clean these glass surfaces in order to keep the illumination at full output. Most algae scrubbers however use discrete (separate) high power LEDs which produce enough illumination in a small space to burn off algal growth on glass surfaces; for these you do not need to wipe the growth off because it does not grow there.

A last difference is overgrowth of algae on the algae itself. Chaeto is a slow growing species of algae because of it’s thick cellular structure, and if conditions favor faster growing algae you will get green hair algae which attaches on top of the chaeto, causing the chaeto to be blocked from light and flow, and eventually causing the chaeto to die and rot. There is no easy way to wipe green hair algae from chaeto; the chaeto must just be harvested earlier instead. For algae scrubbers, green hair algal growth on top of more green hair growth is how scrubbers operate in the first place, so earlier harvesting is not needed.

Nutrients are defined as inorganics, not organics. The word "nutrient" is sometimes confused with "nutrition", and maybe in restaurants the words might mean the same thing, but for aquarists they are totally different. Nutrition food particles are mostly visible, but nutrients are invisible, and for aquariums the nutrients are:

Ammonia/ammoniumUrea (pee)NitriteNitratePhosphateIronCO2

Organics: These are food particles, and most of them big enough to see. They can be apples, pellets, nori, baby brine shrimp, flakes, peanut butter, poop, mucus, leaves, twigs, fish eggs, and other types of detritus, all of which are eaten/consumed by some type of organism. Organics are usually very visible when concentrated, and only after the organics get digested by a long chain of animals and bacteria do organics become invisible inorganics (this is called “remineralisation”, because they are now basic minerals once again). Organic food particles (which include waste) can be large, small, or dissolved, and if dissolved in water then the water may be cloudy or clear. For example, if you take mucus and blend it in water, the resulting dissolved organics would be invisible. Natural reefs are very highly loaded with organics, especially at night (sometimes a night diver cannot see his own hand because of the camera light reflecting off of the mass of particles). Lakes even more so, so much that sometimes you cannot see more than a meter underwater in full daylight. Aquarium keepers however tend to want ultra “clear” water, where all the natural food particles are removed from the water.

Algal Structure: The structure of algal cells make the algae thick or thin; solid or soft. The thinner the algae is, the more surface area it has, just like small particles of sand have more surface area than larger pebbles do. This increased surface area has more contact with water around it and thus can pull in nutrients faster. And the softer the algae is, the less structural cellulose-like material (like celery) it has. Hard structural cells, like celery, are great for holding a shape but bad for photosynthesis because there are less photosynthetic cells like there are in a leaf; so harder/stiffer algae absorb nutrients slower. Therefore for faster nutrient absorption, you want thin and soft algae.

Chaeto: Has a firm structure that holds it's shape, and is about 1 mm in thickness. Nutrient absorption is slow.

Green Hair: Has a soft structure that does not hold its shape, and is about 0.1 mm in thickness. Nutrient absorption is fast.

Slime: A different category altogether.

Light: Photosynthesis does all the nutrient filtering, and it requires light; if the light is reduced, then filtering is reduced. Two facets of algal cells can alter the light: Translucency and self-shading. Translucency is the ability of light to go through a strand of algae; if light can do this, the light can reach cells further inside or on the other side of the strand and do more filtering there. Self-shading is when one strand of algae shades another strand; when this happen to a large degree, the growth of inner portions of a clump of algae slows down or dies, as outer growth is added over it. Thus the clump may appear to be increasing is size but the inner portions will actually be dying and putting nutrients back into the water, sometimes faster than the newer outer layers are taking the nutrients out of the water. And the larger the clump is, the more the inside starts dying. Only the outside portion grows.

Chaeto: Non-translucent (opaque), with high shading of other strands.

Green Hair: Medium to high translucency, with medium shading.

Slime: Low translucency when thick, and high shading.

The graphs of the following study show the light-blocking characteristics of chaeto: "Production within dense mats of the filamentous macroalga Chaetomorpha linum in relation to light and nutrient availability"

Fig 5B shows how, under bright light, chaeto productivity (filtering) drops 72 percent with just 2 cm of chaeto thickness. And this does not take into account any dying chaeto underneath.

With green hair algae however, the green hair filaments are very thin, and translucent, so light and water flow spread throughout the algae, thus maximizing filtering. No part of the algae is "on the dark side of the growth" like it is on almost all parts of chaeto.